1. Field of the Invention
The present invention relates to reducing conducted and radiated electromagnetic interference (EMI) in motor drive applications. More specifically, conducted and radiated EMI is reduced by interposing inductors in ground lines of motor drive circuits, and by arranging components on a printed circuit board to reduce the ground loop area.
2. Description of the Related Art
Legislation limits the amount of high frequency power permitted to be injected into a power supply line by, for example, a motor drive circuit. A known motor drive circuit 2 is illustrated in the block diagram of FIG. 1. Motor drive circuit 2 drives electric motor 4 and includes a rectifier section 6:and an inverter section 8. A capacitor 10 is connected across the connections between rectifier 6 and inverter 8. A heat sink 12 typically is required for inverter 8.
EMI reducing components are added to reduce the outgoing energy as shown in the block diagram of FIG. 2 showing motor drive circuit 20. A common mode filter 22 carries the main power current and thus needs to carry a large current. Consequently, filter 22 must be a large size, and consequently has a high cost. The function of this filter 22 is to reduce the common mode and differential mode current into the system. In addition, filtering capacitors 24, 26, and 27 are added across rectifier 6. No filtering is used in ground line 28.
Radiated EMI is generated in the ground connections between heat sinks and filter capacitors, for example. Typical layouts in which the filter capacitors and power switches are separately located, for example, yield high emissions due to long loops in ground traces.
The present invention provides reduced conducted and radiated EMI in motor drive applications. Conducted EMI is reduced by interposing an inductance in the ground line of a motor drive circuit. The common mode current enters the motor drive circuit through the two supply leads and leaves the system through the ground lead. The ground current typically is very high frequency but very low in rms amperage. Consequently, interposing an induction (low current, very high L) in the ground line advantageously reduces EMI.
Advantageously, the inventive method potentially reduces or eliminates the need for filtering capacitors.across the rectifier of the motor drive circuit. In addition, the inventive method potentially minimizes or eliminates the common mode choke.
Radiated EMI emissions are reduced by sandwiching ground traces along with power traces and connecting to the heat sink with the shortest possible route. This reduces the ground loop area, which consequently reduces emissions.